The present invention relates to methods and apparatus for the production of semiconductor ribbons from a melt and more particularly to improved apparatus and methods in which ribbon edges are controlled and defined by grooves in a moving chill surface.
A reference known to the Applicants and believed to be relevant to the present invention is U.S. Pat. No. 3,605,863 issued to King on Sept. 20, 1971. This patent is hereby incorporated by reference for its general teachings of apparatus and methods for formation of ribbon materials which are generally applicable to formation of semiconductor ribbons.
It is generally recognized that a major portion of the cost of production of solar cells is the cost of the original high purity silicon and its preparation into wafers of suitable size and shape. A standard process involves the growing of monocrystalline boules using the Czochralski technique and then mechanically sawing the boules into thin slices or wafers. Half of the original silicon material is typically lost in the sawing process. Photovoltaic cells have also been made from polycrystalline silicon after it was poured into essentially brick-shaped molds and then sawed into square slices. While such processing of polycrystalline material is less expensive than growth of single crystal material, the resulting cells have lower efficiency and essentially half of the material is still lost in the sawing process. Much effort has, therefore, been made to develop apparatus and techniques for producing wafers or ribbons of silicon directly from the molten state. The above-referenced King patent illustrates one type of apparatus which has resulted from such efforts.
The basic method illustrated by the King patent includes the contacting of a molten body of material with the surface of a moving drum or wheel. The surface of the wheel is cool relative to the molten mass and by cooling a portion thereof, pulls or drags off a film or ribbon of the material in a solidified form. In using this method, it is important to provide a careful balance between the molten material flow to the wheel and wheel speed. If too little material is provided to the surface, full width ribbon will not be formed. It is, therefore, generally considered preferable to run in a slightly flooded condition which means that occasionally molten material will spill over the edge of the wheel. Whenever one of these spills occurs, surface tension effects cause the errant material to drag off a sizeable volume of ribbon which results in a rough jagged ribbon edge. The rough edges must be trimmed to provide uniform width ribbon for subsequent processing. In addition, edge rounding may be required to reduce edge chips and cracks which can cause breakage during later processing. The additional machining and smoothing steps are undesirable since they add to the cost of processing the semiconductor material into wafers or slices.